1. Field of the Invention
This invention relates to circuitry that evaluates and monitors electrical systems, in particular the integrity of a boat's galvanic isolator.
2. Background of the Prior Art
When boats or marine craft are docked and are connected to a dock's AC electric distribution system typically comprised of hot, neutral and grounding conductors, a number of potential dangers exist. Although a boat may be correctly wired, an inadequate or disconnected grounding conductor on the boat, or reversed polarity of the dock's AC electrical distribution system, poses a danger to electrical components on the boat. A more serious danger is that of electric shock to individuals in the boat, or possible death by drowning of individuals in the water. The electric shock hazard exits as a result of inadequate AC grounding systems causing normal stray AC leakage and possible fault currents to flow through exposed metal boat components and through the water or through individuals in contact with an adequate ground.
A boat typically includes AC powered appliances including battery chargers, refrigerators, air conditioners, and the like. A boat battery supplies DC power to DC powered devices such as engine starting motors, navigation equipment, running lights, and the like. A boat's AC safety ground and DC powered devices are bonded at a common point at the boat's DC ground, which is normally the boat's engine. For the purposes of this description, a boat's grounding system is referred to as the AC grounding system. As a boat's AC and DC electrical systems are bonded at a common point, it is understood that the AC grounding system includes a boat's negative DC electrical system.
Boats connected to a dock's electrical distribution system share that system's common earth ground. This common ground acts to electrically connect the boats together. When two or more of these boats are connected via a dock's electrical system, a galvanic cell can be established between different underwater metal parts of these boats. Galvanic action results and what is referred to as DC galvanic current flows through the water between adjacent boats and through the commonly shared AC grounding conductor. This galvanic action can corrode a boat's underwater metal parts, and possibly sink a boat. The degree of corrosion is dependent on a number of parameters, such as the salinity of the water, electric current density, surface area and nature of the underwater metals, etc.
A common device used to block galvanic current is the galvanic isolator. The galvanic isolator is serially installed in the boat's AC grounding conductor, usually immediately adjacent or within close proximity to the boat's power inlet. The galvanic isolator is typically comprised of two sets of opposing pairs of silicon diodes and an optional capacitor in parallel with the two diode sets, which are enclosed in a metal or plastic housing. The threshold voltage at which diodes are biased varies among diodes. Typically all silicon diodes start conducting current at a voltage level as low as 0.4 to 0.45 volts and are assumed to be in full conduction at approximately 0.6 volts to 0.8 volts. Therefore, the diode pairs of a galvanic isolator start conducting at 0.9 to 0.95 volts and are fully biased at 1.20 to 1.6 volts.
The primary purpose of the galvanic isolator is to block all DC galvanic currents in both directions, but provide a path for AC leakage or fault currents back to the dock's power system earth ground. The sets of diodes, and the capacitor, if one is provided, are electrically insulated from the boat. However, one side of the galvanic isolator is connected to the boat's negative engine terminal, or DC ground, which electrically bonds the negative side of the boat's DC system and the AC grounding system together, and its other side is connected by the boat's AC grounding conductor to the AC grounding pin on the boat's power inlet.
Galvanic isolators are relatively inexpensive, compact, and light. However, the integrity of galvanic isolators can be compromised in a number of ways. For example, a faulty open diode or open capacitor of a galvanic isolator would normally prevent or block DC galvanic currents from flowing, but may not pass AC leakage or fault currents back to the dock electrical system. These currents may nevertheless find an alternative path to ground, possibly through the boat's out-drive, through the water and adjacent boats to the dock's earth ground, thus endangering persons in the water and on other boats.
Even if a galvanic isolator is initially operable, limited fault currents may not immediately trip the dock's circuit breaker, but may create high temperatures within the galvanic isolator and pose a fire hazard. The high temperature may cause the galvanic isolator diode(s) to short (or open). A shorted diode compromises the ability of the galvanic isolator to effectively block DC galvanic currents.
Typically, the integrity of a galvanic isolator is not checked until a boat sinks or is significantly damaged by corrosion, or until fire, drowning, or electrocution results. Since galvanic isolators are installed in the AC grounding conductor and used in the boating industry to block DC galvanic current, the integrity of the boat's AC grounding conductor should never be compromised. Therefore, as was recognized by the invention disclosed in U.S. Pat. No. 5,746,008 of Landreth, which is hereby deemed incorporated herein by reference, there is a need in the boat industry for an apparatus that determines if a boat's galvanic isolator is operational, and generates a signal if it is not. The '008 patent disclosed an electrical integrity test system for boats that checks the integrity of galvanic isolators installed on boats connected to shore power. The present invention improves the electrical integrity test system of the '008 patent by recognizing that when a correctly wired boat is connected to the dock's electrical distribution system via the shore cord, occasional variations in the conduction level are encountered during testing of the boat's galvanic isolator. The variations in conduction of correctly wired boats vary from boat to boat, marina to marina, and boat to marina. The present invention further recognizes that the variations in conduction are caused by the entire grounding system to which the boat is connected. A boat's grounding system includes: (1) the boat's negative DC electrical system where the galvanic isolator is connected, which includes the boat's metallic components which have an electrical potential that is determined by the combined composition of the boats metals; and (2) the shore green grounding conductor which is referenced to earth ground at some point on shore, and is connected to the boat' galvanic isolator on the side opposite to which the boat's negative DC electrical system is connected.
Thus, the galvanic isolator in a boat essentially lies between a boat's DC negative electrical system and earth ground. Thus, for a correctly installed galvanic isolator the voltage potential of the earth ground will unlikely be the same as the boat's DC negative potential, due to the presence of galvanic isolator between the two reference points, which creates a slightly different electrical potential between the two. The earth ground and the boat's underwater metallic fittings are both in contact with the water, which serves as the common conductor or electrolyte of the electrical circuit containing the galvanic isolator. This circuit could be viewed and modeled as including two batteries having slightly different potentials connected to opposed sides of the galvanic isolator with the water being the common electrical path between the two batteries.
The differential voltage potential across the galvanic isolator under normal operating conditions is low enough to keep the silicon diodes from conducting. This differential voltage potential of the earth ground to the boat's negative DC ground system varies continuously as a result of (1) changes in the salinity of the water, (2) velocity of the water moving around the boat, (3) number of boats sharing the common earth ground and (4) the combined electrical potential of all the surrounding boat's underwater fittings within the marina connected to the common earth ground.
As explained in detail in the '008 patent and by way of background, to determine the status of the boat's galvanic isolator, the test system, under the control of the embedded micro-controller, conducts a series of test. First the test system applies a voltage across the galvanic isolator with respect to the boat's negative DC system biasing one pair of the silicon diodes into conduction while evaluating the voltage drop across the galvanic isolator. The test system then applies a negative voltage across the galvanic isolator, biasing the remaining pair of diodes into conduction.
It is recognized by the present invention that the varying differential voltage across the galvanic isolator may occasionally make it difficult to bias the pair of diodes to the same conduction level each time during testing. For example, for a galvanic isolator installed in a boat docked at one marina, the test voltage applied across one pair of the galvanic isolator's diodes may generate a voltage drop across the galvanic isolator of 1.50 volts. The same boat, when moved to another marina and connected to its electrical distribution service, under the same testing conditions of the galvanic isolator, the measured voltage drop across the galvanic isolator may be 1.30 volts. Additionally, different voltage drops across galvanic isolators may be seen in different boats next to one another in the same marina. In some cases, the measured voltage drop may be sufficiently lower.
As stated earlier, the biasing voltage characteristics range for a pair of series diodes is approximately 0.9-1.6 volts. If one of the diodes were shorted this voltage range would be approximately 0.45-0.85 volts. When the measured voltage across the galvanic isolator is in the lower voltage conduction level (0.9-1.20 volts), it is more difficult to determine if the measured voltage drop implies the diodes are in a low conduction state and the integrity of the galvanic isolator is intact or if one diode may be shorted. In other words, a single shorted diode is the hardest failed condition to detect when testing a galvanic isolator.
With the foregoing in mind, it becomes an object of the present invention to reliably and accurately determine the difference between a normal voltage drop and voltage drops across a galvanic isolator indicative of galvanic isolator fault.
It is another object of the invention to use two single silicon diodes as reference diodes and means to compare the measured voltage drop across the galvanic isolator's two series diodes and the measured voltage drop across a single reference diode. The single reference diodes may be part of the invention present circuitry as shown in FIG. 1, or two single diodes within the galvanic isolator under test could be utilized as the reference diodes as shown in FIG. 2 and further described herein.